U.S. Pat. No. 5,162,047 discloses a thermal treatment installation which comprises a thermal treatment chamber, a “wafer boat”/rings assembly, a loading device and a transport device. With this arrangement the loading device serves to place wafers in and to remove wafers from the assembly and the transport device serves to place the assembly in and remove the assembly from the thermal treatment chamber.
The assembly consists of a frame to which a large number of rings are joined equally spaced, with their flat sides positioned above one another. The rings are each provided with a recess, which is not specified in more detail, in the inner edge, on which a wafer can be placed by the loading device.
After loading a large number of wafers, the assembly is moved by the transport device to the thermal treatment chamber to subject all wafers, located on the recesses, simultaneously to a treatment in which a heat treatment takes place.
In installations as disclosed in U.S. Pat. No. 5,162,047 the wafers remain in contact with the rings during the entire heat treatment.
During heat treatment of a substrate, for example a silicon wafer, plastic deformation of the wafer can occur. In the case of silicon at temperatures higher than 900–1000° C. the mechanical strength of the wafer decreases substantially and plastic deformation can occur more easily than at room temperature. The deformation of silicon wafers occurs because crystal planes can shift over one another under the influence of stresses present or generated in the material. This is known by the term “slip”. This slip can lead to warping of the wafer such that this is detectable with the naked eye.
Two sources of stress which give rise to slip will be present in the material. Firstly, the force of gravity, which in the case of horizontally positioned wafers is exerted uniformly over the entire surface thereof, in combination with the wafer support, which in general takes place at only a few points. This leads to local mechanical stresses, in particular on and close to the support points, which are also termed gravitational stresses.
Secondly, there is a temperature gradient over the wafer which leads to a non-uniform expansion of the wafer with corresponding mechanical stresses, also referred to as thermal stresses. This temperature gradient over the wafer occurs in particular on introducing it into a reactor and removing it therefrom. In general the temperature in the reactor will be appreciable, for example 900–1000° C., in order to achieve an adequate throughput time. If the ambient temperature is room temperature, on introduction of the wafer into or removal of the wafer from the reactor a substantial temperature gradient will be produced, with the resultant stresses. After all, the thermal capacity is relatively low because of the limited thickness and the large radiating surface of the wafer.
In installations as disclosed in U.S. Pat. No. 5,162,047 the wafers are thus also subjected to a temperature difference during heating and cooling at those locations where there is contact with the ring, since the ring has a certain thermal capacity. So as not to allow temperature differences during loading into and unloading from the thermal treatment installation to become so large that mechanical stresses in the wafer lead to plastic deformation, transport of the combination into and out of the thermal treatment chamber must always take place at a suitable speed.
Moreover, the connection between the rings and the frame gives rise to an additional difference in thermal capacity in the rings which, as a result of the positioning of the connection, can lead to local deviation of the temperature in the ring and the wafer, as a result of which mechanical stresses can also be produced locally in the wafer during heating/cooling. Local adverse deformation of the wafer can occur as a result.
In some installations treatment is not carried out on a large number of wafers at the same time, as in U.S. Pat. No. 5,162,047, but, for reasons specific to the treatment process, only one wafer is treated at a time. For such thermal treatment installations in which only a single wafer is treated per thermal treatment it is customary according to the prior art to place the wafer in, or remove the wafer from, the thermal treatment chamber individually, that is to say directly with the aid of a transport mechanism and without an auxiliary support such as, for example, a wafer ring.